This invention generally relates to electronically controlled motors and to systems, such as heating, ventilating and/or air conditioning systems having motors therein for driving indoor blowers.
Air flow in the indoor section of a heating, ventilating and/or air conditioning (HVAC) system generally determines several key performance characteristics of such a system. For example, in the air conditioning mode, the rate of air flow affects: 1) total system capacity for cooling and dehumidification; 2) latent system capacity for dehumidification; 3) overall efficiency (capacity/input power); and 4) indoor sound level. Variations in the rate of air flow, though cause these performance characteristics to vary differently.
In general, total heating of air conditioning capacity of a system increases as the indoor air flow rate increases. However, upon reaching a particular air flow rate, the blown air will be less warm or less cool, depending on the mode of the HVAC system, than the air in the room. For air conditioning systems, the indoor coil, or evaporator, tends to "saturate". In other words, the coil extracts incrementally less energy from the air for equal incremental changes in air flow. Further, the electric power consumed by the indoor blower motor increases rapidly as the air flow rate increases. The system dissipates electric energy as heat in the same air that is being cooled by the system thereby reducing its net cooling capacity. Due to the effects of coil saturation and blower motor power consumption, net total capacity peaks at a certain air flow rate.
The latent capacity of a system is its ability to remove moisture from the air. Generally, the evaporator coil becomes warmer thereby diminishing its ability to condense moisture as the air flow rate increases. Therefore, latent capacity of the system decreases as air flow rate increases.
A measure of the efficiency of a typical system may be defined by the total capacity divided by the total input power consumed by the system's components, e.g., the indoor blower motor, the compressor and the condensor fan motor. The power drawn by the indoor blower motor increases by the cube of the change in indoor air flow rate. Also, an increase in the air flow rate beyond what is required warms the evaporator coil to produce a higher load on the compressor motor. Thus, increasing air flow increases total power at a growing rate but increases total capacity at a diminishing rate. Efficiency peaks at a certain air flow rate which is less than the air flow rate corresponding to maximum total capacity.
Another concern for HVAC systems is the level of noise in the system. Indoor sound levels are directly related to the rate of air flow. Therefore, minimizing air flow rate minimizes the level of noise in the system.
For these reasons, maximum total capacity of a given system is achieved at a higher rate of indoor air flow than the rate at which maximum efficiency, dehumidification and sound levels are achieved.
Conventional systems, however, provide only a single air flow rate for cooling and another for heating. The indoor blowers are typically driven by discrete speed induction motors. Even in systems having a multispeed induction motor, the installer selects one of the speed taps to operate the motor at single speeds for cooling or heating. In any case, speed changes in an induction motor detrimentally affect system efficiency. More advanced HVAC systems may provide variable speed motors but require a separate controller, such as a humidistat, to switch motor speed.
Since conventional HVAC systems are ill-suited to speed variations, they often lack the necessary total capacity to maintain a desired indoor temperature, as commanded by the thermostat, when the outdoor temperature is extreme. In these cases, the system must operate continuously for prolonged periods of time. When the outdoor temperature is more moderate, though, the system capacity is typically greater than that needed to meet the demand.
Another disadvantage with conventional HVAC systems is that they cycle on and off in response to signals from a thermostat to maintain a set temperature in a conditioned space. However, on and off cycles are generally detrimental to system efficiency. For example, some heat may be transferred to the indoor coil of an air conditioner which remains cold from the preceding on cycle. Conventional systems attempt to recover this residual capacity by delaying turning off the indoor blower fan after the compressor cycles off. However, the system also continues to draw power at the same level as during the cycle and causes additional energy to be consumed in the indoor fan motor. Some conventional systems reduce the air flow rate after the system has cycled off but continue to operate for the duration of the off cycle, also consuming power unnecessarily.